Catalysts for the hydrogenation of substituted arenes to cyclohexanes are known and include standard hydrogenation catalysts such as Pt/Al2O3 and Pd/C, as well as organometallic hydrogenation catalysts, many of which are described by Dyson (Dalton Trans. 2003, 2964-74). Low levels of stereoselectivity have been achieved during hydrogenation of substituted arenes. For example, Russell, et. al. (J. Chem. Soc. Chem. Commun. 1977, 427-8) show 86% selectivity to cis-1,2-dimethylcyclohexane from o-xylene, 79% selectivity to cis-1,3-dimethylcyclohexane from m-xylene and 66% selectivity to cis-1,4-dimethylcyclohexane from p-xylene when using [Rh(η5-C5Me5)Cl2]2 as the hydrogenation catalyst. Hubert, et. al. (Green Chem. 2011, 13, 1766-71) show pressure dependent selectivity to cis-substituted cyclohexanes, obtaining 90% selectivity to cis-1,2-dimethylcyclohexane from o-xylene at 1 bar H2 but 95% selectivity at 30 bar H2, 80% selectivity to cis-1,3-dimethylcyclohexane from m-xylene at 1 bar H2 but 85% selectivity at 30 bar H2 and 64% selectivity to cis-1,4-dimethylcyclohexane from p-xylene at 1 bar H2 but 67% selectivity at 30 bar H2 when using Rh/TiO2 as the hydrogenation catalyst. Toppinen, et. al. (Ind. Eng. Chem. Res. 1996, 35, 4424-33) achieved selectivities of up to 55% to cis-1,2-dimethylcyclohexane from o-xylene, up to 77% to cis-1,3-dimethylcyclohexane from m-xylene, up to 48% to cis-1,4-dimethylcyclohexane from p-xylene, up to 80% to cis-1,3,5-trimethylcyclohexane from mesitylene, and up to 55% to cis-1-isopropyl-4-methylcyclohexane from p-cymene using Ni/Al2O3 as the catalyst. Kubicka, et. al. (Catal. Today 2005, 100, 453-6) show up to 44% selectivity to cis-1,2-diethylcyclohexane from 1,2-diethylbenzene, up to 77% selectivity to cis-1,3-diethylcyclohexane from 1,3-diethylbenzene and up to 55% selectivity to cis-1,4-dimethylcyclohexane from 1,4-diethylcyclohexane when using Ni/Al2O3 as the hydrogenation catalyst.
Muetterties and coworkers (J. Am. Chem. Soc. 1978, 100, 2405-10) have been able to demonstrate high selectivity to cis-substituted cyclohexanes from hydrogenation of substituted arenes, showing the ability to generate cis-substituted cyclohexanes from xylenes and trimethylbenzenes, but they utilize a homogeneous organometallic catalyst, η3-C3H5Co[P(OCH3)3]3. Homogeneous catalysts are difficult to utilize in industrial processes as they are difficult to separate from the products of the reaction and thus to reuse.
Accordingly, it is desirable to provide methods for the hydrogenation of substituted arenes to cis-substituted cyclohexanes with increased selectivity to the cis-isomer. It is further desirable to provide methods for the hydrogenation of substituted arenes to cis-substituted cyclohexanes with selectivities greater than about 80% using a heterogeneous catalyst. Furthermore, other desirable features and characteristics of the methods contemplated herein will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.